Trocars are well known surgical devices. A trocar comprises a stylet, that is, a pointed implement for penetrating a body cavity wall, and associated parts which are surrounded by a tube or cannula which follows the stylet into the cavity and stays in place after the stylet and other associated parts are removed. The combination of trocar and cannula will be referred to herein as a trocar system.
It is known to position protective tubes over the pointed end of the stylet to protect patients and surgeons from injury through inadvertent contact with the sharp tip. Such a tube is described, for example, in U.S. Pat. No. 4,902,280. In this reference, a concentric protective tube is slidably spring biased to fully cover the pointed end. In use, the protective tube moves proximally to expose the pointed end to allow it to puncture the cavity wall. After the pointed end of the stylet and the protective tube pass fully through the wall, the protective tube is able to slide distally under the force of the spring to again cover the pointed end of the stylet to prevent tissue injury.
U.S. Pat. No. 4,535,773 describes an invention wherein the pointed end of a stylet, which protrudes from a protective housing for the purpose of puncturing tissue, also carries pressure sensors which signal the reduction of pressure accompanying full passage through the body wall. This signal causes a solenoid, located within the housing, to become energized and thereby disable a detent which holds the stylet in an extended position, thus allowing a pre-tensioned spring to pull the stylet into the housing.
Although from this cited art it is known to shield the pointed end after the concentric tube has fully passed through the cavity wall, it has now been found to be preferable to invoke protection of the sharp tip at an earlier time, prior to the concentric tube entering or passing through the puncture hole in the process of puncturing the cavity wall in order to further reduce the probability of inadvertently injuring internal tissue. U.S. Pat. No. 4,902,980 requires that the tube fully project through the hole in order to invoke the protective features. Further, while the retraction of the stylet within the housing, as described in U.S. Pat. No. 4,535,773, does improve the level of protection, the impracticality of providing pressure sensors on the stylet and the unreliability of solenoids together with the need to provide electrical power along with the associated costs of these additional parts combine to make this approach impractical.